A key sheet is a component constituting a handy mobile equipment such as a portable phone, personal digital assistant (PDA) or the like, and a plurality of key tops which are push buttons each indicating an alphabetical and/or numerical key or a function key are adhered in an arrayed manner to the surface of a single sheet referred to as a key pad. The above-mentioned key pad is a flexible sheet having rubber elasticity such as a silicone rubber, thermoplastic elastomer or the like. On the undersurface of the key sheet thus configured, a printed circuit board provided with a plurality of normally open contacts closed by pressing a push button is closely disposed, so that push button switches are formed, and further a light source for illuminating a key top from the back surface is disposed.
Since the key sheet is thus configured in a state that a plurality of components are piled up in a layered manner, the thicknesses of each one of components are accumulated to have substantial thickness. Therefore, it is requested that the whole handy mobile equipment should be thinned as much as possible so as to further enhance the portability thereof.
However, for example in the method disclosed in Patent Publication 1, there is a structural limit on further thinner configuration because each component to be piled up has a conventional common configuration. Moreover, there is a limit for the thinning of a key sheet only by using an EL sheet thinner than an LED as a light source. That is, while the EL sheet has a thinner thickness compared with an LED (light-emitting diode) which is conventionally used as a light source in a general way, it requires two sheets of resin films as a substrate sheet forming an EL element consisting of the accumulation of an electrode and a light-emitting layer and as a protective sheet. Therefore, despite the fact that thickness of the EL element itself is very thin, there is a limit for thinner configuration (0.2 mm at the lowest under current situation) due to the addition of the thickness of the above-mentioned two resin sheets.
Patent Reference 1: Japanese Laid-Open Patent [Kokai] Publication No. 2002-050258 A